Porcine Pleuropneumonia
Actinobacillus pleuropneumoniae (thereafter "App") is a Gram-negative bacteria which causes porcine pleuropneumonia, a world-wide distributed infectious disease responsible for great economic losses in the swine industry (Nicolet, J., 1992. In: Leman, A. D., Straw, B., Mengeling, W. L., D'Allaire, S., Taylor, D. J., Eds. Diseases of swine. Ames, Iowa State University Press, pp. 401-408).
On the basis of .beta.-nicotine-adenine dinucleotide requirements (thereafter "NAD"), App can be divided into biotype 1, NAD-dependent strains and biotype 2, NAD-independent strains. So far, 12 serotypes have been described in biotype 1 being serotype 5 often subdivided into 5a and 5b (Nielsen, R., 1986. Acta Vet Scand. 27, 453-455), and 2 serotypes in biotype 2 (Fodor, L., Varga, J., Molnar, E. and Hajtos, I., 1989. Vet. Microbiol. 20: 173-180). Serotyping is mainly based on capsular antigens.
The virulence of the agent seems to be more or less related to the serotype, although all App serotypes described in both biotypes may cause the disease and death in pigs. Biotype 2 strains show less virulence than those of biotype 1, and field observations indicate that serotypes 1, 5a, 5b, 9 and 10 are more virulent than the other biotype 1 serotypes (Dom, P. and Haesebrouck, F., 1992. J. Vet. Med. B., 39: 303-306).
The main route of App spread is airborne and the disease is transmitted mainly by direct contact from pig to pig or by droplets within short distances. In acute outbreaks the infection may jump from one pen to another, suggesting the possible role of aerosols in connection with long distances or the indirect transmission of contaminated exudate by farm personnel. It is generally accepted that App enters the pulmonary alveoli directly via the trachea and bronchi. The clinical signs of porcine pleuropneumonia vary with the state of immunity of the animals, the environmental stress or the degree of exposure to the infectious agent. All age categories may suffer the disease, but young fattening pigs are more frequently affected. The clinical course may be peracute, acute or chronic. In the peracute form, there is an intense inflammatory response, with haemorrhage, edema and fibrinous exudation. The acute form of the disease is characterized by extensive haemorrhage and fibrinous exudation in the pulmonary parenchyma and the pleural cavity, being defined as fibrinous pleuritis and haemorrhagic necrotizing pneumonia that causes the death of the animal within a period of 24-48 hours. Pigs that survive the disease may become carriers of the pathogen, and develop chronic injuries as pulmonary necrosis, nodules and fibrinous adherences in the pleural cavity. Other signs are lack of appetite and decreased gain in body weight (Nicolet, 1992).
Economic losses due to acute outbreaks of the disease result mainly from the high mortality (1-10% or more) and costs for medication (Desrosiers, R., 1986. Vet. Rec. 119: 89-90). Economic losses resulting from chronic cases of pleuropneumonia are due to a decreased growth rate of the affected animals.